1. Field of the Invention
The present invention relates to a light pickup which is used for recording or reproducing information from recording media having different recording densities such as high density optical disks, compact disks and so on.
2. Description of the Related Art
A conventional light pickup for recording and reproducing information from high density optical disks and low density optical disks will be described below. For descriptive convenience, a DVD (digital video disk) and a CD (compact disk) are taken as examples of the high density optical disks and the low density optical disks.
FIGS. 11A and 11B, respectively, are a plan view of the conventional light pickup and a sectional view illustrating main components thereof. In FIGS. 11A and 11B, the reference numeral 50 represents a light pickup for high density optical disks wherein an objective lens 54 for a high density optical disk which condenses a laser beam 53 onto a high density optical disk 52 is cemented and fixed to an objective lens holding cylinder 51. Further, a coil unit 55 which consists of a focus coil and a tracking coil for moving the objective lens holding cylinder 51 in a focusing direction and a tracking direction is cemented and fixed to the objective lens holding cylinder 51. On the other hand, the coil unit 55 is fitted in a permanent magnet and composes a magnetic circuit for driving the objective lens holding cylinder 51 in the focusing direction and the tracking direction. The objective lens holding cylinder 51 is held in a neutral position with an electrically conductive nonmagnetic linear elastic member 57 for supplying electric power to the coil unit 55. The objective lens for high density optical disk has a numerical aperture on the order of 0.6 and a focal length on the order of 3.3 mm.
Explanation will be made of an optical system of the light pickup 50 for high density optical disks which has the configuration described above. The reference numeral 61 designates an optical unit for high density optical disks which comprises a light emitting element and a light receiving element for the laser beam 53 having wavelengths of 635 to 650 nm. The laser beam 53 passes through a collimator lens 62, is made parallel, completely reflected by a surface of a raising mirror 63 coated with multiple layers and condensed by the objective lens 54 for high density optical disks, and forms an optical spot on the high density optical disk 52.
Then, the laser beam 53 which is reflected by the high density optical disk 52 is incident again onto the optical unit 61 for high density optical disks via the path in a direction reverse to that described above, allowed to pass through a diffraction grating (not shown) and received by a light receiving element (not shown). On the basis of optical information which is subjected to photoelectric conversion by the light receiving element, a focus is detected by the known hologram Foutcault and a track is detected by the phase difference method. The objective lens 54 for high density optical disks is always focused on the high density optical disk 52 and controlled so as to follow an information track as described above.
The high density optical disk (DVD) 52 is rotatingly driven by a spindle motor 71.
Then, description will be made of a light pickup for low density optical disks (CD). The reference numeral 70 represents a light pickup for the low density optical disks, which will not be described in particular since it is has a configuration and function which are similar to those of the light pickup 50 for high density optical disks. In an optical system of the light pickup for low density optical disks, the reference numeral 64 designates an optical unit for low density optical disks which comprises a light emitting element and a light receiving element for a laser beam 65 which has a wavelength of 780 nm. The laser beam 65 is completely reflected by a surface of a raising mirror 66, condensed by an objective lens 67 for low density optical disks and forms an optical spot on a low density optical disk 68.
Then, the laser beam 65 which is reflected from the low density optical disk 68 is incident again on the optical unit 64 for low density optical disks via the path in a direction reverse to that described above, allowed to pass through the diffraction grating and received by the light receiving element. On the basis of optical information which is subjected to photoelectric conversion by the light receiving element, a focus is detected by the known hologram Foucault method and a track is detected by the three-beam method. The objective lens 67 for low density optical disks is always focused on the low density optical disk 68 and controlled so as to follow an information track.
The conventional light pickup is configured so as to be capable of recording or reproducing information from an optical disk with the light pickup 50 for high density optical disks and the light pickup 70 for low density optical disks which are composed independently and separately as described above.
However, the conventional light pickup which has the configuration described above poses a problem that it requires a large number of parts, and makes it difficult to configure the light pickups more small-sized and thinner since the light pickup uses two independent optical systems for the light pickup for high density optical disks and the light pickup for low density optical disks. Further, working distances of the two objective lenses described above may be set at 1.5 mm or longer dependently for the sake of optical characteristics and focus control thereof and such long working distances constitute a hindrance to more small-sized configuration of the light pickups.
A primary object of the present invention is to provide a light pickup which is capable of recording or reproducing information from optical disks having different recording densities, and configured to be compact and thin.